We often get fiction manuscripts from writers with a long history of nonfiction and technical books. However, the transition to the world of fiction is not always an easy one. Here are some observations that we have made over years of reading manuscripts:
Know your characters -- Do character sketches including such things as the character's greatest fear and his or her happiest and saddest moments. These are not necessarily revealed within the book but they will help drive a character's response to each situation. Sometimes of course they will be central to the plot when, for example, a character with a fear of heights overcomes that fear in order to triumph.
Remember it is your universe and you create its rules, but once created your universe must obey those rules. Often when reading fiction and plots take improbable twists, when questioned the writer often says, "But that really happened!" Strange and improbable is great for well-documented nonfiction, but unless it conforms to the fictive universe that the writer has created it will likely annoy the reader.
There is a term for plot twists where our hero faces certain death but miraculously escapes unscathed as though the hand of God has intervened -- deus ex machina. Unless the book is whimsical and already playing off of absurd circumstances the reader will likely feel cheated by such a device. Foreshadowing is a better approach. When the character who has been secretly following the hero turns out to be on her side and steps in to help, the reader is more likely to say, "Oh, now that makes sense." Prepare the reader and the reader will accept your premise -- fool the reader and you will lose him.
The great Russian novels would probably not get published in the United States today. A writer has about 30 pages to hook the reader -- including agents and editors. The book that "really picks up on page 150," will sadly languish in the hands of its creator -- however well it may be written otherwise.
And finally we cannot over-emphasize the avoidance of abstractions. This applies to cover letters as well as books. It was once said that a great love poem should not contain the word "love." It is the same with well-written books. They don't tell the reader how a character feels, but rather, allow the reader to feel it through the presentation of the luminous details of a character's life.